1. Field of the Invention
The present invention relates to a motor control device for driving and controlling a motor.
2. Description of Related Art
In order to control a motor by supplying three-phase AC power to the motor, it is necessary to detect current values of two phases (e.g., U-phase current and V-phase current) among three phases including U-phase, V-phase and W-phase. Although two current sensors (current transformers or the like) are usually used for detecting current values of two phases, the use of two current sensors causes an increase of cost of the entire system equipped with the motor.
For this reason, there is proposed a conventional method in which bus current (DC current) between an inverter and a DC power supply is sensed by a single current sensor, and current values of two phases are detected from the sensed bus current. This method is also called a single shunt current detecting method.
FIG. 23 shows a general block diagram of a conventional motor driving system in which the single shunt current detecting method is adopted. An inverter (PWM inverter) 202 is equipped with half bridge circuits for three phases, each of which includes an upper arm and a lower arm, and it converts a DC voltage from a DC power supply 204 into a three-phase AC voltage by switching the individual arms in accordance with specified three-phase voltage values given by a controller 203. The three-phase AC voltage is supplied to a three-phase permanent magnet synchronous motor 201, so that the motor 201 is driven and controlled.
A line connecting the individual lower arms in the inverter 202 with the DC power supply 204 is called a bus line 213. A current sensor 205 transmits a signal indicating bus current that flows in the bus line 213 to the controller 203. The controller 203 does sampling of an output signal of the current sensor 205 at appropriate timing so as to detect phase current of a phase in which a voltage level becomes a maximum value (maximum phase) and phase current of a phase in which a voltage level becomes a minimum value (minimum phase), i.e., current values of two phases.
If voltage levels between different phase voltages are separated from each other sufficiently, current values of two phases can be detected from the output signal of the current sensor 205. However, if the maximum phase and an intermediate phase of voltage become close to each other, or if the minimum phase and the intermediate phase of voltage become close to each other, it is difficult to detect current values of two phases (note that description of the single shunt current detecting method including description of a reason why it becomes difficult to detect current values of two phases will also appear later with reference to FIG. 4 and the like).
Therefore, there is proposed a method in which if current values of two phases cannot be detected by the single shunt current detecting method in a certain period, a pulse width of a PWM signal for each arm in the inverter is corrected based on gate signals of three phases in the period.
A usual correction example of a specified voltage value (pulse width) that also supports the above-mentioned correction is shown in FIG. 24. In FIG. 24, the horizontal axis indicates time while 220u, 220v and 220w indicate voltage levels of the U-phase, the V-phase and the W-phase, respectively. Since a voltage level of each phase follows the specified voltage value (pulse widths) for each phase, they are considered to be equivalent. As shown in FIG. 24, the specified voltage value (pulse width) of each phase is corrected so that “maximum phase and intermediate phase” as well as “minimum phase and intermediate phase” of the voltage do not approach each other closer than a predetermined distance. Thus, voltages of individual phases do not become close to each other to the extent that current values of two phases cannot be detected, and current values of two phases can be detected stably.
When this type of voltage correction is performed, it is necessary to determine a correction quantity based on a relationship among specified voltage values (pulse widths) of three phases. If an applied voltage is low, in particular, the correction may be required for each of the three phases resulting in a complicated correction process.
On the other hand, a direct torque control is proposed as the control method that enables to reduce a torque ripple more than the case of performing a vector control. FIG. 25 illustrates a block diagram of a general structure of the conventional motor driving system that realizes the direct torque control.
In the motor driving system shown in FIG. 25, an α-axis component φα and a β-axis component φβ of a magnetic flux linkage of an armature winding of the motor are estimated based on two-phase current values (iα and iβ) obtained from two current sensors and two-phase voltage values (vα and vβ) obtained from two voltage detectors, and a generated torque T of the motor is estimated. In addition, a phase θS  of the magnetic flux linkage vector viewed from the α-axis is calculated. These values are calculated in accordance with the equations (A-1) to (A-4) below. Here, Ra represents a resistance value of a one phase of the armature winding, PN represents the number of pole pairs of the motor, and “φα|t=0” and “φβ|t=0” represent values of the φα and φβ at time t=0 (i.e., initial values of φα and φβ ), respectively.φα=∫(vα−Raiα)dt+φα|t=0   (A-1)φβ=∫(vβ−Raiβ)dt+φβ|t=0   (A-2)θS=tan−1(φβ/φα)   (A-3)T=PN(φαiβ−φβiα)   (A-4)
Then, target values φα* and φβ* of φα and φβ are calculated based on a torque error ΔT(i.e., T*−T) between the estimated torque T and a specified torque value T*, a phase θS, and a target value |φS*| of a magnitude of the magnetic flux linkage vector made up of φα and φβ (i.e., amplitude of the magnetic flux linkage), and magnetic flux control is performed so that φα and φβ follow φα* and φβ*. More specifically, the target values (vα* and vβ*) of the two-phase voltage values (vα and vβ) are calculated so that φα and φβ follow φα* and φβ*, and the specified three-phase voltage values (vu*, vv* and vw*) are generated from the target values of the two-phase voltage and are supplied to the inverter.
In the direct torque control, voltage and current to be used for estimating the magnetic flux are usually detected (sensed) by a detector (sensor). It is possible to calculate the voltage to be used for estimating the magnetic flux by using the specified voltage value. However, when trying to use the single shunt current detecting method for the direct torque control, it is necessary to correct the specified voltage value. Unless this is taken into account for making up the control system, the estimated magnetic flux will have an error.
The same is true on the case where the single shunt current detecting method is used for the motor control device performing estimation of the magnetic flux as well as the case where the single shunt current detecting method is used for the direct torque control.